1. Field of the Invention
This invention relates to a process and apparatus useful for welding thermoplastic material.
2. Description of the Prior Art
Plastic welding is commonly used to weld together the walls of a crack in a plastic article or to form a connecting weld along a junction line to join two plastic parts. (The word "workpiece" used hereafter is to be understood to encompass the article to be repaired and the parts to be joined.)
Heretofore, the procedure commonly used has involved the following:
usually forming a V-shaped joint or channel along the crack or junction line, to provide space for the filler material and to remove surface oxides; PA0 buffing or rubbing with sandpaper the surfaces of the workpiece weld area and the filler rod, to remove surface oxides; PA0 pre-heating the weld area (i.e. the sloped surfaces of the channel) to welding temperature (450.degree.-800.degree. F.) using a hot gas jet; PA0 and immediately thereafter laying a rod or strip of filler plastic into the heated channel and at the same time applying a jet of hot gas to heat the portion of filler material being laid in, to the welding temperature, with the result that the filler material fuses with the workpiece material to form the weld. PA0 (1) There is a high incidence of defective welds produced. In some cases, the weld does not hold at all, in other cases it is too weak to provide acceptable performance in use; PA0 (2) The process is slow and labor-intensive, thereby making it costly. For example, the requirements: PA0 all result in labour consumption with accompanying cost; and PA0 (3) The use of hot gas to effect softening of the filler and workpiece materials is inefficient. It is difficult to localize the hot gas sufficiently, so that only that portion of the workpiece that requires softening is softened. It has been calculated that the provision of hot gas consumes about 1/3 of the total expenditure used to make the weld. PA0 forcefully delivering the molten filler material through a submerged orifice into the molten workpiece material produces a forced intermingling that yields a strong weld; PA0 the hot tip ploughs through a layer of the oxidized plastic material along each of the edges of the crack or junction line and disperses at least some of it away from the weld area, thereby improving the possibility of obtaining a strong weld; PA0 by using a heated tip instead of hot gas to supply heat to the workpiece plastic in the weld area, localization and efficiency of heating is improved and the cost of the weld is significantly reduced; PA0 because molten filler material is supplied to the weld area, it is capable of distributing itself in the channel - by using a thicker filler rod, it is thus possible to completely fill the channel in one or two passes and thereby shorten the time to produce the weld. PA0 In another aspect of the invention there is provided a novel dispensing device or gun for welding plastic. This gun comprises the combination of: PA0 a barrel having an outlet nozzle, said barrel forming a chamber, said nozzle forming a restrictive orifice for the ejection of molten filler plastic from the chamber; PA0 means for heating the barrel and nozzle, whereby plastic in the chamber may be indirectly heated to a molten condition and the tip of the nozzle may be maintained at a high temperature so that it will heat the workpiece plastic in the weld area to a molten condition, when drawn through said weld area; and PA0 means for forcing filler plastic rod axially into the chamber at a point spaced from the nozzle, to assist in maintaining a high pressure at the orifice outlet.
Prior art United States patents which are exemplary of this technique are U.S. Pat. Nos. 2,722,964 (Duncan), 3,853,669 (Westlein), 3,047,050 (Sourber), and 2,862,543 (Kaminsky).
In applicant's view, there are a number of problems associated with this prior art technique. More particularly:
that the crack or junction line be "V'ed" out before welding; PA1 that pass after pass of small diameter filler rod be laid into the V channel, to fill it from the apex up; and PA1 that the filler rod and workpiece weld area be cleaned by buffing before welding;
There has therefore long existed a need to provide a welding process that addresses these shortcomings and is improved with respect to them. The present invention is directed to satisfying this need.